1. Field of the Invention
The invention relates to a method of producing an optical imaging system, in particular a microlithographic projection objective, which has a plurality of optical elements, and to an optical imaging system produced with the aid of the method.
2. Description of the Related Art
In optical imaging systems which have a plurality of optical elements, the total imaging errors result from the sum of the errors of the individual optical elements contributing to the imaging. Since error tolerances for individual components cannot be reduced arbitrarily, as a rule, fine adjustment of the overall system is required in order to minimize the total error of the system. Such an adjustment process is very complicated, for example in high-performance projection objectives for microlithography. The projection objectives of this type, which not infrequently include more than ten or twenty optical elements, are used, as is known, in projection exposure installations for the production of semiconductor components and other finely structured components and serve to project patterns of photo masks or graduated plates (masks, reticles) at high resolution on a reducing scale onto an object coated with a light-sensitive layer. The required imaging performance with resolutions in the submicron range simply cannot be achieved in these complex systems without complicated adjustment.
A fine adjustment process generally includes a plurality of different manipulations on lenses or other optical elements. This includes lateral displacements of the elements at right angles to the optical axis (designated centering here), displacements of elements along the optical axis for the purpose of changing air spacings (designated tuning here) and/or rotations of elements about the optical axis (designated compensation here). Tilting of individual elements may possibly also be provided. The adjustment procedure is carried out under the control of a suitable aberration measurement of the imaging system, in order to check the effects of the manipulations and to be able to derive instructions for further adjustment steps.
Even after complicated adjustment, residual errors may remain, which can either be eliminated only with considerably increased adjustment effort or not at all by means of adjustment. If the errors exceed the specifications predefined for the optical system, further measures are needed in order to improve the imaging performance. One measure is the introduction of what are known as “correction aspheres” into the optical imaging system. In this way, residual errors which may possibly be present can be minimized further. A “correction asphere” in the sense of this application is an aspherically curved surface of a lens or of a mirror whose surface shape is specifically used to compensate partially or wholly for fabrication errors of an optical system. In this sense, correction aspheres must be distinguished from what are known as “design aspheres”, whose surface shape is defined in the context of the original optical design. Correction aspheres with a deformation between about 10 nm and about 1 μm, which are also designated nanoaspheres or nanometer aspheres are normally used. The use of such aspheres for correction purposes in diffraction-limited high-performance optics is described, for example, in the article “Nanometer-Aspharen: Wie herstellen und wofür?” [“Nanometer Aspheres: How to Manufacture and for What Applications”] by C. Hofman, A. Leitel, K. Merkel, B. Retschke, Feinwerktechnik und Messtechnik 99 (1991) 10, pages 437 to 440.
In U.S. Pat. No. 6,268,903 B1 (corresponding to EP 724 199 B1), an adjustment method for an optical imaging method is described, for which a correction element is fabricated on the basis of a distortion measurement. For this purpose, a correction element, which is part of the projection objective, is provided at a predetermined location in the imaging system. Following a measurement of the distortion of the system, the topography of the surface of the correction element which is required in order to eliminate the corresponding distortion component is calculated. Then, the correction element is removed from the projection system and the correction surface is processed. The correction element is then inserted again. The projection system has an objective part in front of its aperture stop plane and an objective part after this aperture stop plane. The intention is installation positions which are located as far as possible from the aperture stop plane and very close to the object or image. With this, the intention is for the spot size of the small bundle of radiation on the correction surface to be very small, and therefore for the influence on other aberrations to be slight. The installation positions close to the field are also intended to simplify the removal and installation of the correction element. The measurement method used for the distortion is an indirect method, in which a test reticle is imaged onto a wafer coated with photoresist, the exposed wafer is then developed and the imaged pattern is measured with the aid of a coordinate measuring machine.
U.S. Pat. No. 5,392,119 (cf. also WO 96/07075) describes a method for correcting aberrations of an optical imaging system in which at least one imaging error, for example distortion, imaging field curvature, spherical aberration, coma or astigmatism, is measured on the imaging system. On the basis of the measurements, correction plates matched individually for the imaging system are fabricated, their correction surfaces being used to minimize the measured imaging errors. In this way, “spectacles” can subsequently be fitted to an imaging system. As a result, the imaging performance of existing imaging systems can be improved. In one exemplary embodiment, two correction plates are arranged outside the projection objective, between object plane and objective, and one plate in the region of the aperture stop plane. The measuring technique used for determining the imaging errors is a variant of the Hartmann method. In this case, the wavefront errors produced by the imaging system are converted into lateral deviations of the actual image points from the positions of ideal image points which would be achievable with an error-free imaging system. If the measurement is carried out for a plurality of field points, then the deformation of the wavefront in the pupil of the imaging system can be reconstructed on the basis of a model from the field of the resultant deviation vectors. A description of an appropriate measuring technique will be found, for example, in U.S. Pat. No. 5,828,455.
European Patent Application EP 1 022 617 describes a microlithography projection objective in which the last optical element provided in front of the image plane is a correction plate of constant thickness, whose two surfaces have an identical aspherical shape. The shape of the aspheres was determined on the basis of a distortion measurement of the objective.
Japanese Patent Application JP 10-154 657 describes a production process for a microlithography projection objective, in which the lenses of the objective are displaced axially, decentered and/or tilted with respect to one another in order to minimize aberrations, and in which a correction asphere is produced on an optical surface in order to correct aberrations of higher order.